The overall goal of the proposed research is to extend the genetic analysis of the development of the free-living nematode Caenorhabditis elegans, a model organism for studying the genetic basis of animal development and behavior, chosen because of its limited size and cell number, its rigidly specified development, and its suitability for genetic manipulation. An understanding of the genetic basis of development may well be fundamental to much of medicine and may ultimately contribute important information to many problems, ranging from congenital defects to senescence. More specific objectives are: first, to continue the analysis of genetic mosaics produced by the somatic loss of free chromosome fragments in order to localize the anatomical foci of action of various genes known to affect animal behavior, development or anatomy; second, to identify and characterize gynandromorphs in which some cells are XX (normally hermaphrodite) and other cells are XO (normally male); third, to identify and characterize new chromosome duplications, both free and translocated and both X duplications and autosomal duplications, which will be used both to balance recessive lethal and sterile mutations and to generate genetic mosaics; fourth, to tag several genes that affect nervous system development with the transposon Tc1 and to make use of the Tc1 tag to clone at least one of these genes; and fifth, to continue manufacturing, in addition to chromosome duplications, rearranged chromosomes that can act as dominant crossover suppressors, which can be used to balance recessive lethal and sterile mutations.